Soldiers' faces predict aggression, military rank and number of children

New research suggests male soldiers' faces may predict their military rank and how many children they ultimately father

Suomen Rintamamiehet ("Finland's Front Line Soldiers").Image: John Loehr, University of Helsinki.

What role has aggression played in human evolution? Can scientists predict who might be more aggressive? And how might increased aggressiveness be linked to overall fitness? Previous research has found that aggression in hockey players is correlated with their facial width to height ratio (fWHR), and a new study builds on this work by demonstrating that fWHR may also predict a soldier's military rank and how many children he fathers. Below the jump, the authors of this latest study – primarily John Loehr – discuss their work.

Like many people, I read the interesting 2008 study that investigated the relationship between aggression and facial structure in hockey players [doi:10.1098/rspb.2008.0873]. In this study, co-authors Justin Carré and Cheryl McCormick combined psychological testing and real-life data collected during varsity and professional NHL hockey players' careers to demonstrate that there is a connection between aggression in men and their fWHR.

The fWHR consists of two standard measurements used by anthropologists to measure skulls, but the breakthrough that accounts for the current interest in fWHR was the idea that this measurement can also be made (albeit approximately) from a photograph (see right; doi:10.1098/rspb.2008.0873). This makes fWHR special because evolutionary psychologists are not only able to use it to test hypotheses in the lab, but they can also use it to analyse human behaviour in real life situations – in corporate boardrooms [doi:10.1177/0956797611418838], in sports [i.e.; doi:10.1098/rsbl.2013.0140] and even using forensic statistics [doi:10.1016/j.evolhumbehav.2012.02.002]. Studies of the human face have long been done using qualitative scores, in which judges rate faces on subjects' dominance, beauty or masculinity, etc. fWHR provides a quantitative measure, thus removing subjective opinions from the equation.

It is important to point out that there is plenty of variation between individuals, and if an individual has a comparatively wide face it does not mean that this person will necessarily be aggressive. In fact, this particular individual may or may not be more aggressive than a thinner faced man. For this reason, it would be folly to label an individual as "aggressive" based solely on a fWHR measurement, in the same way that it would be wrong to label an individual as "aggressive" solely based on his gender. What we do know is that, on average, men have higher testosterone levels than women and, on average, they tend to be more aggressive than women, but we also know that behaviour is the product of much more than just one's sex or fWHR measurement. From the perspective of our research, what is important is that there are trends in the population that we can identify, and these trends may provide some new insights into human evolution.

Although the relationship between NHL hockey players' penalties and fWHR turned out to be not as strong as initially thought, the original paper has been very influential by inspiring more work in this area. At this time, research shows that fWHR looks more and more useful as a proxy measure for the effects of testosterone and aggression [doi:10.1016/j.evolhumbehav.2013.03.005]. Using this measure, a window is opening that provides us with insight into the roles of aggression and testosterone in human societies.

After reading the paper about hockey players, I wanted to know if fWHR may be predictive for aggression among soldiers? Since war is one of the most aggressive and protracted activities that human populations engage in, I also wanted to know if fWHR might also predict military rank achieved and the number of children fathered by each soldier? To answer these questions, my colleague, biostatistician Bob O'Hara, and I analysed the wealth of images contained in two Finnish publications, Suomen Rintamamiehet (Finland's Front Line Soldiers) and Vapautemme Hinta (The Price of Our Freedom).

It's not everyday when I come across a large amount of data that can help us understand more about ourselves as human beings while also shedding light on deeper evolutionary questions. But this was foremost in my mind when a librarian at the Finnish National Defence University Library pointed to the rows of books containing thousands of Finnish soldiers' photographs and records of their personal information. These books, compiled voluntarily by the soldiers themselves as well as by the families of those who died in war, are treasures. Their importance lies not only in communicating the sorrow of those who lost loved ones in these conflicts but are a way for veterans of those wars to tell their stories and to help them reconnect with comrades with whom they lost contact. Further, by compiling these volumes, these individuals left a unique record that can be used to increase our understanding for how our species behaves and provide insight into the evolutionary forces at work.

Suomen Rintamamiehet is a series of volumes that collect the personal information of Finnish soldiers who fought in the second world war. The pages are filled with surprisingly detailed records, including photographs of the soldiers; the regiments with which they served; the soldiers' birth dates along with those of their wives and children; marriage date and even their children's names.

The other publication, Vapautemme Hinta, is a compilation of the names and photographs of the soldiers who died during the Winter War of 1939-40 against the Soviet Union. Unlike Suomen Rintamamiehet, Vapautemme Hinta contains little personal detail other than a photograph, name and date of death, but it is filled with page after page of the faces of more than 20,000 young men whose lives ended abruptly in that brutal three-and-a-half-months long campaign. As the title of the book implies, the lives of these young men were the price of freedom in a war that became one of the defining moments in Finland's history; this small country united against seemingly insurmountable odds managed to retain its independence (although some territory was indeed lost).

Thanks to the dedication of the Finnish National Archives, we had access to the final piece of the puzzle to make our research possible: the records of the young men whose photographs appear in Vapautemme Hinta, including the number of children they fathered, have been digitised and can be searched online at the Finnish National Archives website.

A plot of how the expected number of children changes with face width height ratio (fWHR). The range of fWHR is the range in the data.

In our study we found, for the first time, that the number of children a man fathers during his lifetime is positively correlated with fWHR. Wider-faced males, on average, have more children, although there is a great deal of variation between individuals. These data suggest that either wider-faced males are favoured by females, or they invest more effort in reproduction.

This correlation supports the evolutionary viewpoint that aggression and testosterone are sexually selected traits. A similar result between the width to height ratio and reproductive success has been found in a historic Austrian population, but in this case the number of individuals was relatively few, and the potential for other variables influencing the result was high (the skulls represent a nearly 300-year-period), which were not controlled for in the analysis [doi:10.1371/journal.pone.0052317.t001]. However, it is still encouraging that the same result has been independently found in two data sets.

Our proxy measure shows no correlation between survival and aggression. From an evolutionary perspective it is perhaps surprising that aggression is not selected for in a conflict within our species. Shouldn't there be a fitness benefit of aggression for some, if one of the products of aggression, i.e., war has been such an essential feature of human culture for millennia? The answer may lie in the advent of technology. Perhaps when conflicts were settled using hand-to-hand combat, a certain type of soldier survived better, but this selection pressure disappeared as technology became more and more important to conflicts. Basically, modern warfare leaves little room for an individual to determine his own fate, and each soldier is at the mercy of chance whether he happens to have a shell drop into the trench he is in. It is also possible that war only provides certain individuals with a fitness advantage when an army invades new territory, and conquering troops father children with the women living there.

One finding that still has me scratching my head is the correlation between military rank and fWHR. Previous research has found that aggressiveness and social dominance are positively correlated, but our data reveal that officers were thinner-faced than their subordinates.

A plot showing how rank changes with fWHR, showing that soldiers with wider faces tended to have a lower rank. To read the graph, pick a fWHR, and draw a vertical line. The proportion of the line that is in the red zone is the probability that a soldier with that fWHR would be enlisted. The grey shading shows the uncertainty around the lines.

There are several possible explanations for this. The military ranks that we used in the data analysis were from the beginning of the war, and it is possible that these ranks changed changed during the war. But even if it turns out that the correlation between rank and fWHR changed after the war started, this leads us to ask why did the dominance structure of the military initially form in this way? Do other militaries have the same dominance structure?

It is possible that wider-faced soldiers may be better cooperators than thin-faced soldiers: the evidence suggests that wider-faced males are better cooperators than thinner-faced males in competitive situations [doi:10.1177/0956797611435133]. Differences in trustworthiness may also play a role. This same study also suggests that thinner-faced males are more trusted and tend to abuse their power less often. However, this said, the story underlying the formation of this particular dominance structure is probably much more complicated.

The proxy measure seen in male faces is an excellent tool to explore the role of aggression in human evolution, but at the same time it is only a proxy. How different would our results be if we were able to directly measure aggression or testosterone is a question that, a least for the time being, remains unanswered. However, fWHR appears to provide some fascinating insights into the evolutionary role of human aggression that we can use to formulate new hypotheses to test. What is certain is that through the power of the scientific process we will come to know ourselves much better.

GrrlScientist is an evolutionary biologist, ornithologist and science writer who writes about the interface between evolution, ethology and ecology, especially in birds. You can follow Grrlscientist's work on facebook, Google +, LinkedIn, Pinterest and of course, on twitter: @GrrlScientist